This invention relates to spread-spectrum communications systems that utilize Gold-code sequence generator in combination with an M-ary encoding scheme.
A conventional mobile communication system comprises communication networks each of which includes a base station and a number of mobile radios that communicate with the base station. In such a communication system, a communication signal from one mobile radio should not interfere with communication signals from other mobile radios within a network, and communication signals from one network should be free of interference from communication signals of other networks. Conventionally, a Time Division Multiple Access (TDMA) scheme is utilized to separate one mobile radio signal from other radio signals within a network. Further, different networks may then separate themselves from each other by using a Frequency Division Multiple Access (FDMA) scheme.
When a communication network uses the TDMA method, it is desirable for receivers in the mobile radios to synchronize to a signal transmitted from the base station quickly since the time expended for synchronization is time that cannot be used for communication. Thus, non-coherent modulation techniques are preferable since they do not require additional time to synchronize, and since they are more tolerant of Rayleigh-fading channels. However, non-coherent modulation schemes are more susceptible to interference than coherent modulation schemes. M-ary signaling is one non-coherent modulation scheme that minimizes the susceptibility to interference. For example, 64-ary orthogonal signaling is used in the IS-95 CDMA cellular system. One weakness with this form of modulation is that the orthogonality among signals is lost when the signals are transmitted through a multipath channel, since the delayed component of the signal is identical to another signal.
With respect to FDMA, as the number of networks increases within a given geophysical area, the degree of separation achieved with the FDMA scheme is reduced. This results in one network interfering with the networks that use adjacent frequency signals. This interference is most detrimental when a radio causing the interference is closer to a base station than a mobile radio that actually belongs to the base station""s network. This phenomenon is called the xe2x80x9cnear-farxe2x80x9d interference problem. This problem is somewhat mitigated by confining the frequency spectrum of the communication signals for each network. Gaussian-filtered, Minimum Shift Keyed (GMSK) modulation is one method of containing the spectral characteristics of the communication signals. However, this approach does not entirely solve the near-far interference problem.
Therefore it is desirable to provide a mobile communication system that allows multiple communication networks to operate within the same physical area with little or no interference among the communication signals from different networks and among different mobile radios within one network.
The present invention allows a wireless communication system to maximize the number of co-existing networks within a frequency band while achieving a large far-near ratio, and while using inexpensive class-C transmitters. This is achieved in part by using spreading codes that retain their near-orthogonality when transmitted through multipath channels.
Morever, the present invention permits a plurality of radios to access the network by time-sharing the transmission channel in such a manner that a receiver can synchronize to each transmission with a minimum of time, which in turn allows simultaneous communication of voice and data information.
The present invention also allows multiple private conversations between mobile radios while a plurality of the mobile radios receive broadcast data.
More specifically, the present invention provides a communication device that includes a transmitter and receiver. The transmitter includes an M-ary encoder configured to generate an Mxe2x88x921 number of distinctive symbols, each comprising k bits. M is equal to 2k and k is a positive integer. The transmitter also includes a code generator configured to produce spread spectrum codeword sequences based on the symbols generated by the M-ary encoder and based on first and second Gold code polynomials. The transmitter is configured to send a radio signal based on the spread spectrum codeword sequences. The receiver is configured to receive the radio signal. The receiver includes a first shift register configured to receive an input signal generated based on the received radio signal and a second shift register configured to receive and circularly shift a locally generated codeword sequence, identical to codeword sequence used to encode symbols. The receiver also includes an accumulator coupled to the first and second shift registers and configured to multiply and accumulate stored values in the first and second shift registers each time the second shift register is circularly shifted and a selecting device coupled to the accumulator and configured to identify one symbol from the plurality of symbols based on outputs from the accumulator. A method corresponding to the above described communication device is also provided.
In another aspect of the present invention, the base station and its mobile radios share the carrier frequency assignment by dividing transmission time into segments called time-slots. The time-slots are organized into frames such that mobile units and the base station can be assigned one or more time-slots within a frame, during which time they may transmit their modulated signal as a message segment. Each time-slot assignment can be used to convey voice or data information.
The intended recipient, or recipients, for each message segment are identified within each message segment by its xe2x80x9cdestination address.xe2x80x9d This enables the base station to broadcast messages to a plurality of mobile units while several mobile-base-mobile transmissions occur. Each mobile-base-mobile communication consists of the mobile unit transmitting a message segment to the base station, which then re-transmits the message segment to another mobile unit. When appropriate, the re-transmission from the base station may be addressed to a plurality of mobile units. Message segments may contain either voice or data information.
Time-slots may also be assigned to a plurality of mobile units for infrequent transmissions. In this case, the mobile units share the assignment using a slotted-ALOHA protocol. If two mobile units transmit simultaneously, their signals will not be correctly received and both units retransmit the message segments after a random delay if they are not acknowledged by the base station.
The base station and mobile units demodulate the spread-spectrum message segments by first synchronizing the receiver""s time reference to a preamble spreading code. Using the synchronized time reference, the receiver demodulates M-ary symbols by finding the one-of-M spreading sequence which correlates best with the received signal. The M-ary symbols are then decoded into binary data conveying either voice or data information. The present invention includes a simplified method of finding the sequence with highest correlation.
Furthermore, the mobile radios of the present invention can transmit and receive digitized voice data or Internet communication packets. In other words, when a computer is connected, the mobile radio of the present invention can send and receive messages to and from the Internet. This capability is in addition to operating as a regular mobile telephone.
In addition, mobile radios, which are capable of synchronizing the frequency of their respective oscillators to their respective base-stations to reduce the likelihood of introducing demodulation errors in both the base-to-mobile and mobile-to-base communication links, are also discussed.